Process and apparatus for producing elongated, particularly tape-shaped semiconductor bodies from a semiconductor melt



1966 w. SPIELMANN ETAL 3,

PROCESS AND APPARATUS FOR PRODUCING ELONGATED, PARTICULARLY TAPE-SHAPED SEMICONDUCTOR BODIES FROM A SEMICONDUCTOR MELT Filed Sept. 20, 1961 United States Patent 3,293,001 PROCESS AND APPARATUS FOR PRODUCING ELONGATED, PARTICULARLY TAPE-SHAPED SEMICONDUCTOR BODIES FROM A SEMI- CONDUCTOR MELT Werner Spielmann, Munich, Giinther Ziegler, Erlangen, and Walter Heywang, Munich, Germany, assignors to Siemens & Halske Aktiengesellschaft, Berlin, Germany, a corporation of Germany Filed Sept. 20, 1961, Ser. No. 139,400 Claims priority, application Germany, Sept. 20, 1960, S 70,427 12 Claims. (Cl. 23-301) Our invention relates to a process and apparatus for the production of long, particularly tape-shaped, semiconductor bodies from a melt of the semiconductor material supercooled below the melting point, according to which a semiconductor body is dendritically grown onto a crystal seed and is pulled out of the melt.

According to known processes of this type, the semiconductor body is pulled out of a melt contained in a crucible, thus entailing inevitable contamination of the semiconductor material by material from the crucible wall, this contamination being particularly disadvantageous when high-melting semiconductor materials, for example, silicon, are involved.

It has also become known to obviate these disadvantages of a crucible by supporting the melt in form of a mound on the upper end of a thick semiconductor rod and pulling continuously from the melt with the aid of a seed crystal, a monocrystal which is rather thin in comparison with the supporting semiconductor rod. During the pulling operation, new semiconductor material can be continuously supplied to the melt by causing adjacent parts of the melt-supporting semiconductor rod to become liquid. In this known method, the melt on top of the thick semiconductor rod was maintained throughout above the melting temperature of the semiconductor material. For dendritic growth of a monocrystal, however, the crystal must be pulled from a supercooled melt. Therefore, particularly with the usually preferred inductive heating and the resulting inductive stirring of the molten material, this known method at best resulted in an irregular growth of many dendrites, mainly occurring at the boundary between the melt and the supporting semiconductor rod.

We have discovered, and have established by comprehensive tests, that in spite of past experience and in spite of past expectations, it is possible to pull perfect dendrites out of a crucible-free melting zone by providing for a particular design of the heating device and a corresponding control of the simultaneous cooling, employing a seed crystal to which the dendritically grown material becomes attached.

More specifically, we support a melt of semiconductor material upon a carrier consisting of the same semiconductor material and having a relatively large cross section. We heat the melt on top of the carrier by a heating device in an irregular manner, namely so that we apply more heat and obtain a higher temperature in the portion of the melt adjacent to the carrier than in the portion more remote from the melt. We thus maintain the portions of the melt heated to a lesser extent at a temperature just below the melting point of the semiconductor material (about C.). As a result, dendritic semiconductor material will grow on the seed immersed into the relatively cool portion of the melt which is then pulled out of the melt, while simultaneously maintaining the temperature in the portion of the melt closer to the supporting solid semiconductor body slightly above the melting temperature. According to the method of the invention, therefore, a high temperature gradient is Patented Dec. 20, 1966 "ice produced and maintained within the melt by maintaining only a portion of the melt above the melting temperature Whereas another portion of the melt is kept at a temperature, just below (about 10 C.) the melting temperature, although the latter portion, from which the drendrite is being pulled, remains liquid and in supercooled condition.

For maintaining only a portion of the melt in abovedescribed manner at a temperature below the melting temperature of the semiconductor material, it is preferable to provide additional means or expedients for reducing the amount of heat produced per unit of time in this portion of the melt by the heating device. When employing inductive heating, this can be done, for example, by placing a good conducting short-circuiting ring of metal into the vicinity of the melt portion to be kept relatively cool, so that the ring, on account of its shortcircuiting action, suppresses the flow of induction currents in the melt portion to be kept cool. However, if the melt is heated by other means, for example by heat radiation, it is preferable to shield the portion of the surface to be kept cool, thus impeding or preventing the impinging of heat radiation at this location. In addition, or in lieu of these expedients, the heat dissipation at the surface portions of the melt to be kept cool can be made greater that the heat dissipation at the other portions of the melt surface. This can be done, for example, by directing a current of gas against the surface portion of the melt to be subjected to supercooling.

When performing the method according to the invention, a difficulty may arise when inserting the seed crystal at which the dendritic growth of semiconductor material is to occur, into supercooled portions of the melt without causing an uncontrollable growth of dendrites to commence in the melt when the seed is being immersed. For avoiding this difliculty, and according to another feature of our invention, the melt is preferably first heated throughout to a temperature above the melting point, thereafter the seed is immersed into the melt, and only thereafter the portions of the melt adjacent to the seed are cooled below the melting temperature in the abovedescribed manner. As a result, there first occurs a state of thermal equilibrium in which no material grows into the melt from the seed and on the other hand no material crystallizes onto the seed out of the molten material. When thereafter the temperature of the melt in the vicinity of the seed decreases sufiiciently below the melting temperature, the dendritic growth of the semiconductor material at the seed crystal commences. Such growth, however, would soon cease, if the seed crystal were not pulled away from the melt. However, when the melt is pulled away in accordance with the rate of dendritic growth of the semiconductor material, the material growing onto the seed is continuously removed from the melt. In order to maintain this growing operation, and in accordance with another feature of our invention, we provide for a corresponding replenishment of semiconductor material to the melt as the latter is being gradually depleted. Such replenishment of material into the melt is preferably effected in the manner, known in principle from Zone-melting operations, by continuously melting material from the melt-carrying semiconductor body, which then becomes part of the melt.

Doping material can, when a doped dendrite is desired, be added to the melt or preferably a doped semiconductor carrier used to support the melt. The carrier can be either monocrystalline or polycrystalline.

The invention will be further described with reference to the apparatus schematically illustrated by way of example on the accompanying drawing in which:

FIG. 1 shows a vertical sectional view of the apparatus; and

FIG. 2 shows separately the melting-zone portion and induction heater.

Denoted by 1 in FIG. 1 is the carrier body of semiconductor material, preferably silicon, which may have a rod-shaped or disc-shaped design. A portion of the semiconductor material of carrier 1 is kept molten during operation of the apparatus by means of a heating device 4. The melt 2 thus formed is supported as a mound on top of the solid portion of the carrier body 1. In the illustrated example, the heating device comprises an induction coil 4 which is adjustable in known manner along the semiconductor body 1. The device is further provided with an electromagnetic supporting or levitating field coil 6 which, like the induction-heater coil 4, is displaceable along the semiconductor body 1 in the upward and downward direction as indicated by a double-headed arrow 6'. By means of the electromagnetic field of coil 6, the melt is supported and thus prevented from dripping ofit'. The use of such electromagnetic supporting or levitating fields for the support of a floating cruciblefree melt is known, per se. surrounded by a cooling ring 7, which can likewise be displaced longitudinally with respect to the carrier body 1 as is indicated by a double-headed arrow 7'. The cooling ring 7 permits controlling or regulating the melting of the material of carrier body 1 depending upon the positional adjustment of the cooling ring 7. Located above the melt is a short-circuiting ring which surrounds the dendritically grown semiconductor body 3' continuously pulled out of the melt. With a sufiiciently close coupling with the upper portions of the melt, the

ring 5 coacts with the induction currents generated in the melt by the heater coil 4 to strongly Weaken the induction currents in the immediate vicinity of the location where the semiconductor body 3' enters into the melt. The cooling ring 7 consists preferably of a copper tube which is connected to a supply of liquid coolant such as water. The coils 4 and 6 may. also consist of copper tubing and may likewise be traversed by coolant.

The semiconductor portion 3' dendritically grown onto the seed crystal 3 is continuously pulled out of the melt by means of a transporting device 8, 9, here shown to consist of two nipping rollers 8 and 9. The position of the heater coil 4, adjustable upwardly and downwardly as indicated by the double-headed arrow 4', is preferably so chosen that it essentially causes heating only of the outer peripheral portions in the melt 2 near the boundary face of the supporting carrier body 1. In addition, the short-circuiting coil 5 maintains the induction currents in the upper portions of the melt at a small value. When The semiconductor body is carriers being schematically. indicated herein bythe dou-' the portions 2" of the melt adjacent to the solid portion the upper portions 2' of the melt 2, into which the semiconductor body 3' is immersed, have a temperature below the melting temperature of the semiconductor material, then new material will continuously grow onto the semiconductor body 3, which is being continuously pulled out of the melt by the transporting device 8, 9. In order to permit this continuous growth of the body 3' to persist, new semiconductor material is continuously supplied to the melt. For this purpose, the heating device 4 is being displaced downwardly with respect to the carrier body 1 at such a speed that the amount of materials thus melting from the body 1 into the melt is approximately equal to the quantity of material simultaneously being pulled out of the melt during the dendritic growing operation.

The supporting field coil 6 and the cooling ring 7 must travel downwardly simultaneously with the coil 4; and the short-circuiting ring 5 is continuously kept at an approximately constant distance from the top of the molten mound. The supporting field coil 6, cooling ring 7, coil 4, and short-circuiting ring 5 may be mounted on carriers such as shown in Emeis et al. Patent No. 2,904,663 and Emeis application Serial No. 409,610, filed February 11, 1954, now Patent No. 3,030,194, which type carriers are incorporated herein by reference, the movement of said of the carrier body 1 continuously maintain a temperature above the melting point despite the traveling motion of the melt, and the portions 2' of the melt adjacent to the growing dendritic body 3' remain at a temperature below the melting point.

As illustrated, a carrier body 1 of hyperpure silicon is mounted vertically. on a holder 1' located on the base If desired, the holder 1',may

plate 10 of the apparatus. 7 be displaceable upwardly with respect to the base plate 10. The wall 11 of the processing vessel is closed at thetop by a cover plate 12. The sealed interior 13 of the vessel is filled with a neutral gas, e.g., argon, or preferably with a reducing gas of good heat conductance, particularly with hydrogen. It is preferable to pass the hydrogen, H during the crystal-pulling operation through the vessel 13 in order to take advantage of the gas flow in this manner for regulating the cooling conditions for the melt 2 so that the above-described temperature distribution is maintained in the melt. For this purpose, the gas is supplied to the vessel 13 in relatively cool condition, for example at normal room temperature, and its rate of flow is adjusted to the desired value. The gas inlet and outlet openings are respectively denoted by 13 and 14. i

The cover plate .12 has a centrally located opening .12.

for the passage of the seed crystal 3 or for the dendritic semiconductor body 3, which becomes attached during the growing operation to the seed crystal. Since the cross section of the dendritically grown semiconductor body 3' is preferably considerably smaller than the cross section of the supporting semiconductor body 1, a very long semiconductor band or tape structure 3 can be pulled out of the relatively short vessel 13 in this manner. This permits producing a sufiiciently thin semiconductor body 3 in form of a tape which can be wound upon a spool or roll outside of the processing vessel 13.

In lieu of a supporting field coil 6 and an additional cooling pipe 7, the preferably water-cooled pipe 7 can simultaneously serve as supporting coil, as is the case in the modified embodiment illustrated in FIG. 2. In this embodiment, the tubular ring denoted by 6, 7 is traversed by alternating current of suitable frequency for imposing a levitating field upon the melt. The tubular ring 6, 7 is cooled so intensively as is required for sufiiciently cooling the carrier body 1. FIG. 2 further relates to the starting stage of the process in which no material has as yet grown onto the crystal seed 3 immersed into the melt 2.. The preferably likewise water-cooled short-circuiting coil 5 is still relatively far remote from the top of the molten mound 2 because, at the beginning of the process and as described above, the seed 3 is first immersed into a melt that is heated throughout to a temperature above the melting point. By placing the coil 5, which may be on a separate support, closer to the top of the melt 2, as is' indicated by an arrow 5", the temperature of the molten 7 flow of hydrogen that passes by this part in the processing 7 vessel 13; in addition, or in lieu of this expedient, the area of the melt 2 about the immersion point of the seed 3 can be cooled by blowing or similar means, down to the desired temperature. As soon as the temperature has been reduced sufliciently, the dendritic growth of semiconductor material from the melt onto the seed will commence, and the seed is now pulled out of the melt in accordance with the rate of growth.

The process according to the invention can be modified in various ways. For example, the semiconductor body 1 can be suspended from the top portion of the vessel so that the melt is suspended from its bottom end. As mentioned, the coils 4, 5, 6, 7 are displaceable relative to the carrier body 1; however, it is preferable to make them also displaceable relative to each other. This is particularly favorable and in some cases necessary with respect to the heater coil 4 and the short-circulating coil 5 as shown in FIG. 2, because then the process can be initiated in a particularly simple manner. The electromagnetic supporting field coil 6 or the levitating field produced thereby are not absolutely necessary but can be omitted. It is, however, essential that the adjustment of the coil 4, the regulation of the heat produced by coil 4 in the melt, as well as the cooling of the portions 2' in the melt from which the semiconductor 3 is pulled, are carefully controlled during performance of the process. It is important that after the process is initiated, the temperatures of the melt portions 2 and 2" are continuously maintained below and above respectively the melting point of the semiconductor material and that simultaneously the pulling speed of the grown body 3' and the travelling speed of the heater coil 4 and of the other coils 5, 6, 7 relative to the carrier body 1 are controlled in accordance with the temperature conditions obtaining in the melt 2. It is advisable to use as a crystal seed 3 a monocrystal whose Ill-axis is parallel to the surface of the melt at the immersion point of the seed. The wall 11 of the processing vessel 13 consists preferably of quartz and is preferably well cooled during operation by cooling water, preferably by passing a flow of water directly along the vessel wall, in order to thus promote the dissipation of heat required or supercooling the portion 2 of the melt.

While specific reference has been made to silicon, it is obvious that the process is equally applicable to other semiconductor materials, such as germanium or A B compounds, as defined by Wellrer in US. Patent No. 2,798,989. Silicon melts at 1420 C. and is supercooled by about C. during the pulling operation. The supercooling for other semiconductor materials is also in the order of about 10 C.

We claim:

1. The process of producing a long, particularly tapeshaped, dendritic semiconductor body, which comprises the steps of holding a crucible-free melt of the semiconductor material adhering to the top of an upright carrier consisting of the same material, heating the melt substantially to a temperature above the melting point of the material in a portion of the melt that is adjacent to the carrier by applying a heating eflFect from without the melt, simultaneously maintaining a less heated other portion of the melt at a temperature below said melting point, and continuously pulling with the aid of a crystal seed, a dendritically growing semiconductor body out of said other portion of the melt while continuously maintaining said other portion supercooled and said carrieradjacent portion above said melting temperature.

2. The process of producing a long, particularly tapeshaped, dendritic semiconductor body, which comprises the steps of holding a crucible-free melt of the semiconductor material adhering to the top of an upright carrier consisting of the same material, heating the melt substantially to a temperature above the melting point of the material in a portion of the melt that is adjacent to the carrier by applying a heating effect from without the melt, simultaneously maintaining a less heated other portion of the melt at a temperature below said melting point, continuously pulling, with the aid of a crystal seed, a dendritically growing semiconductor body out of said other portion of the melt while continuously maintaining said other portion supercooled and said carrier-adjacent portion above said melting temperature, and supplying new semiconductor material to the melt in a quantity which corresponds approximately to the quantity being pulled from the melt as a dendritic body.

3. The process of producing a long, doped, dendritic semiconductor body, which comprises the steps of holding a crucible-free melt of the semiconductor material adhering to the top of an upright carrier consisting of the same material, heating the melt substantially to a temperature above the melting point of the material in a portion of the melt that is adjacent to the carrier by applying a heating effect from without the melt, simultaneously maintaining a less heated other portion of the melt at a temperature below said melting point, continuously pulling, with the aid of a crystal seed, a dendritically growing semiconductor body out of said other portion of the melt while continuously maintaining said other portion supercooled and said carrier-adjacent portion above said melting temperature, and supplying new semiconductor material, and doping material to the melt in a quantity which corresponds approximately to the quantity being pulled from the melt as a dendritic body.

4. The process of producing a long, particularly tape shaped, dendritic semiconductor body, which comprises the steps of holding a crucible-free melt of the semiconductor material adhering to the top of an upright carrier consisting of the same material, said carrier being of polycrystalline hyperpure semiconductor material, heating the melt substantially to a temperature above the melting point of the material in a portion of the melt that is adjacent to the carrier by applying a heating effect from without the melt, simultaneously maintaining a less heated other portion of the melt at a temperature below said melting point, and continuously pulling, with the aid of a crystal seed, a dendritically growing semiconductor body out of said other portion of the melt while continuously maintaining said other portion supercooled and said carrier-adjacent portion above said melting temperature.

5. The process of producing a long, tape-shaped, doped dendritic semiconductor body, which comprises the steps of holding a crucible-free melt of the semiconductor material adhering to the top of an upright carrier consisting of the same material, said carrier being of doped semiconductor material, heating the melt substantially to a temperature above the melting point of the material in a portion of the melt that is adjacent tothe carrier by applying a heating elfect from without the melt, simultaneously maintaining a less heated other portion of themelt at a temperature below said melting point, and continuously pulling, with the aid of a crystal seed, a dendritically growing semiconductor body out of said other portion of the melt while continuously maintaining said other portion supercooled and said carrier- Iadjacent portion above said melting temperature.

6. The process of producing a long, particularly tapeshaped, dendritic semiconductor body, which comprises the steps of holding a crucible-free melt of the semiconductor material adhering to the top of an upright carrier consisting of the same material, inductively heating the melt substantially to a temperature above the melt-ing point of the material in a portion of the melt that is adjacent to the carrier by applying an inductive heating effect from without the melt, simultaneously maintaining a less heated other portion of the melt at a temperature below said melting point, continuously pulling with the aid of a crystal seed a dendritically growing semiconductor body out of said other portion of the melt while continuously maintaining said other portion supercooled and said carrier-adjacent portion above said melting temperature, displacing said inductive heating effect longitudinally along the carrier at a rate that said inductive heating effect melts a quantity of the carrier approximately equal to the quantity being pulled from the melt.

7. The process of producing a dendritic semiconductor body, which comprises the steps of electromagnetically holding a crucible-free melt of said material to an upright carrier consisting of the same material, heating the melt substantially to a temperature above the melting point of the material in a portion of the melt that is adjacent to the carrier by applying a heating effect from without the melt, simultaneously maintaining a less heated other portion of the melt at a temperature below 7 said melting point, continuously pulling with the aid of a crystal seed .a .dendritically growing semiconductor body out of said other portion of the melt while continuously maintaining said other portion supercooled and said carrier-adjacent portion above said melting temperature, displacing said heating effect longitudinally along the carrier at :a rate that said heating effect melts a quantity of the carrier approximately equal to the quantity being pulled from the melt.

8. The process of producing a dendritic semiconductor body, which comprises the steps of electromagnetically holding a-cruci-blefree melt of said material to an uplight carrier consisting of the same material, heating the melt substantially to a temperature above the melting point of the material in a portion of the melt that is adjacent to the carrier by applying a heating effect from without the melt, simultaneously maintaining a less heated other portion of the melt at a temperature below said melting point, continuously pulling with the aid of a crystal seed a dendritically growing semiconductor body outof said other portion of the melt while continuously maintaining said other portion supercooled and said carrier-adjacent portion above said melting temperature, displacing said heating effect longitudinally along the carrier at a rate that said heating effect melts a quantity of the carrier approximately equal to the quantity being pulled from the melt, and applying a cooling elfect .to the carrier, said cooling elfect being separated from said melt by said heating effect.

9. The process of producing a dendritic silicon semiconductor body, which comprises heating the upper end of an upright silicon carrier to form a melt thereof, electromagnetically holding said melt to said carrier, immersing.

a silicon seed crystal into said melt, cooling the melt in the immediate vicinity of said seed to about 1440 C., pulling a dendritically growing silicon semiconductor body from said melt at a rate equal to that of the dendritic growth of said seed, while maintaining the portion of the melt in the vicinity of said silicon carrier at a temperature above 1420 C., and the portion of the melt in the vicinity of the growing dendrite at a temperature about 1410 C., and supplying new semiconductor material to the melt in a quantity which corresponds approximately to the quantity being pulled from the melt as a dendritic body.

10. The process of producing a dendritic silicon semiconductor body, which comprises heating the upper end of an upright silicon carrier to form amelt thereof, electromagnetically holding said melt to said carrier, immersing a silicon seed crystal into said melt, cooling the melt in the immediate vicinity of said seed slightly below 1420 C., which corresponds to the melting temperature of silicon, by reducing the heating effect in this portion of the melt, pulling a dendritically growing .silicon semiconductor body from said melt :at a rate equal to that of the dendritic growth of said seed, while maintaining the portion of the melt in the vicinity of said silicon carrier at a temperature above 1420 C., which corresponds to the melting point of silicon, and the portion of the melt in the vicinity of the growing dendrite at a temperature slightly below the melting point of silicon, and supplying new semiconductor material to the melt in a quantity which corresponds approximately to the quantity being pulled from the melt as a dendritic body.

11. The process of producing a dendritic silicon semiconductor body, which comprises heating the upper end of an upright silicon carrier to form a melt thereof, electromagnetically holding said melt to said carrier, immersing a silicon seed crystal into said melt, cooling the melt in the immediate vicinity of said seed between about 1410 C. and 1420 C. by cooling the surface portions of the melt by :a gas current, pulling a den-dritically growing silicon semiconductor body from said melt at a rate equal to that of the dendritic growth of said seed, while maintaining the portion of the melt in the vicinty of said silicon carrier at a temperature above 1420 C. and the portion of the melt in the vicinity of the growing dendrite at a temperature between about 1410" C. and 1420 C., and supplying new semiconductor material to the melt in a quantity which corresponds approximately to the quantity being pulled from the melt as a dendritic body.

12. Apparatus for producing a dendritic semiconductor body, which comprises an upright crucible free semiconductor carrier, means to melt substantially to a temperature above the melting point of the material a portion of the melt that is adjacent to the carrier by applying a heating effect from without the melt, electromagnetic supporting means for supporting said melt, inserting and withdrawing means for inserting a seed crystal into said melt and subsequently withdrawing said seed crystal and growth thereon, means for simultaneously maintaining a less heated other portion of the melt at a temperature below said melting point, said means being positioned in the vicinity of said seed crystal to maintain the melt in this vicinity at a supercooled temperature to effect dendritic growth, each of said heating, supporting and cooling means being coaxial with said carrier and being longitudinally displaceable along said carrier.

References (Iited by the Examiner UNITED STATES PATENTS 3/1959 France.

OTHER REFERENCES Bennett et al.: Dendritic Growth 'of Germanium Crystals, Physical Review, vol. 116, No. 1, Oct. 1, 1959, pp.

Butterworths: Scientific Publications, 1958, pp. 194- 196.

E. Billig: Growth of Monocrystals of Germanium From an Undercooled Melt.

' Hannay: Semiconductors, Reinhold Publishing Company, Feb. 27, 1959 (pp. 114 to Proceeding of theRoyal Society A, vol. 229, 1955, pp. 346363.

Lawson and Neilson: Preparation of Single Crystals, London.

NORMAN YUDKOFF, Primary Examiner.

MAURICE A. BRINDISI, Examiner.

A. J. ADAMCIK, G. P. HINES, Assistant Examiners. 

1. THE PROCESS OF PRODUCING A LONG, PARTICULARLY TAPESHAPED, DENDRITIC SEMICONDUCTOR BODY, WHICH COMPRISES THE STEPS OF HOLDING A CRUCIBLE-FREE MELT OF THE SEMICONDUCTOR MATERIAL ADHERING TO THE TOP OF AN UPRIGHT CARRIER CONSISTING OF THE SAME MATERIAL, HEATING THE MELT SUBSTANTIALLY TO A TEMPERATURE ABOVE THE MELTING POINT OF THE MATERIAL IN A PORTION OF THE MELT THAT IS ADJACENT TO THE CARRIER BY APPLYING A HEATING EFFECT FROM WITHOUT THE MELT, SIMULTANEOUSLY MAINTAINING A LESS HEATED OTHER PORTION OF THE MELT AT A TEMPRATURE BELOW SAID MELTING POINT, AND CONTINUOUSLY PULLING WITH THE AID OF A CRYSTAL SEED, A DENDRITICALLY GROWING SEMICONDUCTOR BODY OUT OF SAID OTHER PORTION OF THE MELT WHILE CONTINUOUSLY MAINTAINING SAID OTHER PORTION SUPERCOOLED AND SAID CARRIERADJACENT PORTION ABOVE SAID MELTING TEMPRATURE. 